Darien tells Lamont he's a cop and Lamont asks him to prove it. Darien then says he can't because his badge is in his stolen wallet. This is so far from the truth; Darien could have proven it a number of ways, the least of which calling his precinct.
Since Selma projects sounds from her, the sound of police sirens and her voice pretending to be a cop, should grow more distant and echo off buildings as the thieves make a fast getaway, but they don't.
When Mitch removes the sword from the wall, the back of the sword becomes visible and clearly a wooden prop.